In the late 1980s, when I was in the Eastern Province of Saudi Arabia, a friend suggested that I drive out into the desert near Jubail to see the oldest extant Christian church in the world. And there it was, surrounded by a chain-link fence to keep casual visitors and foreign archaeologists out. Experts who saw the site before it was closed said that the church was built by Nestorian Christians, and was probably used from the 4th to the 9th century. Its existence embarrassed the Saudi government, which prefers to believe that Arabia went straight from paganism to Islam....